Increasingly stringent statutory regulations relating to permitted pollutant emissions in motor vehicles, in which internal combustion engines are arranged, mean that pollutant emissions during operation of the internal combustion engine have to be kept as low as possible. This can be achieved on the one hand by reducing the pollutant emissions which result in the respective cylinders during combustion of the air/fuel mixture. On the other hand exhaust gas post-treatment systems are deployed in internal combustion engines to convert the pollutant emissions generated in the respective cylinder during the combustion process of the air/fuel mixture to harmless substances. Three-way catalytic converters are deployed as exhaust gas catalytic converters in petrol combustion engines in particular. Highly efficient conversion of the pollutant components, such as carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx), requires a precisely set air/fuel ratio in the cylinders.
The composition of the mixture upstream of the exhaust gas catalytic converter must also exhibit a predefined fluctuation and so specific operation of the internal combustion engine is necessary, both with an excess of air and a shortage of air, to fill and empty an oxygen storage unit of the exhaust gas catalytic converter. When oxygen is introduced into the oxygen storage unit, nitrogen oxides in particular are reduced, while when the oxygen storage unit is emptied, oxidation is assisted and stored oxygen molecules are prevented from deactivating sub-regions of the exhaust gas catalytic converter.
The oxygen storage unit is for example also configured to store oxygen for very short periods and to bind it or release it as required. It has a surface storage unit and a deep storage unit for storing oxygen.
A lambda control and a trim control assigned to it for petrol-operated internal combustion engines is known from the specialist publication “Handbuch Verbrennungsmotoren” (Internal combustion engine handbook), published by Richard von Basshuysen/Fred Schäfer, 2nd edition, June 2002, Friedrich Vieweg und Sohn Verlagsgesellschaft mbH Braunschweig/Wiesbaden, pages 559-561. The lambda control is intended to ensure that the pollutant components CO, HC and NOx are converted as efficiently as possible, in conjunction with the deployment of an exhaust gas catalytic converter configured as a three-way catalytic converter.
The lambda control includes the upward modulation of a forced stimulation to a setpoint value of the air/fuel ratio to optimize the efficiency of the catalytic converter. An actual value of the air/fuel ratio is determined as a function of the signal from a linear lambda probe upstream of the exhaust gas catalytic converter and thus a control deviation is determined for the lambda controller, which is configured as a PII2D controller, and a correction value for correcting a quantity of fuel to be metered in is determined at its output. The quantity of fuel to be metered in, which has been corrected by means of the correction value, is metered into the combustion chambers of the cylinders by way of the fuel injection valves.
The trim controller is configured as a PI controller, which utilizes the signal of the probe downstream of the catalytic converter, which is less exposed to cross-sensitivities.
It is known from the same specialist publication, pages 568 ff., that the catalytic converter can be monitored. The oxygen storage capacity of the catalytic converter is used for this purpose, correlating with hydrocarbon conversion in the exhaust gas catalytic converter. Greater forced stimulation is deployed for catalytic converter diagnosis. In the case of a new exhaust gas catalytic converter with a relatively high oxygen storage capacity these control fluctuations are significantly attenuated and the probe signal downstream of the exhaust gas catalytic converter therefore only has a small fluctuation amplitude. An older catalytic converter has a significantly poorer storage behavior, so that the fluctuation present before the exhaust gas catalytic converter has a correspondingly greater impact on the exhaust gas probe downstream of the catalytic converter. To monitor the catalytic converter therefore the signal amplitudes of the lambda probes in front of and behind the catalytic converter are evaluated and the quotient of the amplitudes is then formed. This amplitude ratio is used to assess the conversion rate of the catalytic converter.
There is an increasing requirement to monitor for possible nitrogen oxide emissions as well and in particular also to estimate NOx emissions.